I've used toilet paper for a lot of things around the house. When mixed with paint or glue it is as hard as wood when dry. I've used it in my older house where the cabinet shelves have pulled a half inch or so away from the walls. Mixed the same paint I was using on the boards with toilet paper to make a dough like mass, pressed and smoothed it into the crevices and it is still hard after 30 or so years. Can hardly tell where the shelf ends and the wall begins.
I suspect that with the water, a touch of Elmer's glue would make these strong and more permanent. I like them. I wonder if adding a touch of food coloring to the water might make them a soft color that will go with the "old fashioned" look that I love from memories of my childhood. Probably have to put down a piece of plastic wrap first, to keep it from glueing itself to the mold permanently.